Treating metal surfaces to increase the coefficient of friction



'tates This invention relates to a new and improved adhesion treatment of metal surfaces and more particularly to a method and composition :for increasing the coefficient of fn'ction between metal surfaces capable of motion one with respect to the other. The invention is especially Con- Cerned with the provision of Chemical means for preventing Wheel slippage between locomotive Wheels and rails. However, in its broader aspects the invention contemplates the irnprovement of frictional Contact between metal suraces which are susceptible of slippage one with respect to the other, as, for example, the slippage which occurs between a shaft and a sheave, gear or pinion rictionally mounted on said shaft.

The past several decades have produced railway locomotives possessing great power and weight, thus enabling long, heavily-laden trains to be pulled by single en gines. With the advent of the extremely heavy locomotive it was felt that driving Wheel slip would be eliminated. It was soon discovered, however, that the increased static weight carried on the driving -wheels did not solve the problem to any great extent. Track sanding techniques were developed but this only partially alleviated the condition. Wheel slippage has proven to be an erratic condition which has not in all cases been satisfactorily explained.

In one explanation of the problem, rail slip is said to result from a tough invisible oil film on the wear band of the rail. Trafiic and heat destroy this film and high adhesion results. When a light rain occurs or when the rails reach the dew point, as the result of the relatively high humidity, a water film forms across the wear band where it may contact oil deposits on the edge of a rail with the result that a film of oil creeps through and replaces the water film. The oil deposits on the rail sides act as reservoirs for the formation of new Oil films and water acts as the transporting agent. The oil deposits on the rail come from journal box oil leakage by way of the outside face and outer portion of the tread of the car Wheels. There are other sources of contamination such as road crossings, rail lubricators, and the like.

The importance of solving the problem is strikingly illustrated when it is realized that only of the engines weight can be utilized as tractive force When the rails are greasy and moist, and 30% when the rails are clean, dry and sanded. Even a small improvement in these figures, as expressed in the terms of increased coeflicient of riction, would enable railway locomotives to operate more efficiently and economically as Well as providing improved braking for railway locomotives and rolling stock.

Another instance where it is desirable to improve the frictional contact between two metal surfaces capable of motion one with respect to the other is where a gear or pinion is frictionally mounted on a shaft. obviously, if slippage occurs between the gear or pinion and the shaft the efliciency of the particular operation in question is reduced or the device may even become inoperable.

It is therefore an object of this invention to provide a new and improved method for enhancing the frictonal contact between two metal surfaces capable of motion one with respect to the other.

Another object of this invention is to provide a method for raising the coefficient of friction between railway car Wheels and rails.

Still another object is to raise the coeificient of ris i i atet ice 2 tion between railway car Wheels and rails having an oil film thereon.

A further object is to provide a method of decreasing slippage between railway Wheels and rails.

Another 'object is to provide a Chemical treatment to prevent locomotive slippage on dry, wet, or oily rails.

Still a further object is to enable railroad locomotives to utilize more of their tractive forces on wet oily rails than -has heretof-ore been considered possible.

An additional object of the invention is to produce a railway rail containing an adherent coating of a material which substantially prevents slippage between the rail and a locomotive or railway car Wheel.

Stll a further object is to provide a method of substantial-ly preventing slippage between the beariug surfaces of a railway rail and a locomotive or a railway car Wheel by applying to the rail and/ or Wheel a solid of colloidal fineness which acts as a slip inhibiting substance.

A further and more specific object of the invention is to provide a new and improved method for improving the frictional contact between a metal shaft and a sheave, gear or pinion frictionally mounted thereon.. Other objects Will appear hereinafter.

In accordance with the invention it has been found that the coeflicient of friction between metal surfaces capable of motion one with respect to the Other can be increased by applying to at least one of the Contacting surfaces a .fine hydrous metal oxide having a. surface area of at least 25 square meters per gram.

The hydrous metal oxide is preferably applied to the metal surface in a non-lubricating hydrophobic liquid, preferably in a non-lubricating hydrophobic liquid Which is capable of 'being volatilized. However, the fine hydrous metal oxide can be applied as a suspension or sol in a hydrophilic liquid such as water, an alcohol or an ether alcohol, e. g., methyl alcohol, ethyl alcohol, isopropyl alcohol, ethylene glycol, diethylene glycol, ethylene glycol monobutylether ethylene glycol monoethylether, and the like. The hydrous metal Oxide can also be applied as a fine dried hydrous metal Oxide obtained, for example, by drying a hydrous metal oxide sol or by other methods wherein a ve1y finely divided hydrous metal Oxide is produced. Hydrous metal oxides which are especially suitable for the invention are those which are capable of forming stable suspensions or sols in non-lubricating hydrophilic or hydrophobic liquids having a viscosity at 26 C. of not greater than 35 centipoises and wherein the concentration of finely divided hydrous metal Oxide is at least 5% by weight of the resultant suspension or sol.

For the purpose of the invention the hydrous metal Oxide can also be mixed with Oxygen derivatives of nonmetallic elements such as silica. Such a mixture is a byproduct of the manufacture of catalysts usually referred to as catalyst fines." One typical example of such a mixture contains 13% Al O and 87% SiO Another example contains 25% Al O and SiO The specific surface area of such catalyst fines is usually within the range of 450 to 600 square meters per grarn. Ground catalyst fines of the type described which have been ground to predominant average particle size within the range of 0.1 to l micron with a maximum particle size not greater than about 5 to 10 microns are very suitable for the practice of the invention.

Other hydrous metal oxides which are suitable for the practice of the invention are the hydrous metal oxides having physical Characteristics of the type above described which are oxides of iron (ferric and errous), cobalt, nickel, chromium, beryllium, tin, Zinc, manganese, vanadium, molybdenum, antimony and tungsten. Such metal oxides can be prepared in the form of sols by various methods which are well known.

Two general methods employed for preparing such sols are: (l) condensation (or precipitation) whereby smaller particles are formed or built up to colloidal dirnensions; and (2) dispersions (or peptization) whereby larger particles are broken down to colloidal diniensions. According to one method the metal Oxide sol can be produced as described in Bird, US. 2244325, by passing an allzali metal salt of an acidic metal Oxide through a cation exchange resin in the hydrogen form whereby the effiuent consists of the corresponding hydrous metal Oxide sol. Thus, dilute Solutions of salts, such as sodium tungstate, sodium molybdate, potassium pyroantimoniate, sodium vanadate and sodium germanate can be passed through a cation exchanger to produce the corresponding sol. An example of a suitable cation exchanger is a resin of the type described in U'.S. Patent 2,3 66,007, one form of which is sold Commercially as Nalcite HCR. This resin is a sulfonated cross linked styrene-divinylbenzene resin.

An alternative method of producing sols of hydrous metal oxides which is useful for producing some types of metal oxides which cannot be produced by the Bird method is described in Ryznar, U.S. 1438230, where a dilute solution of a water soluble salt containing the desired metal cation is contacted with an anion exchanger in the hydroxide forn] and the resultant hydrous metal Oxide is produced. By using this method, sols of aluminum, iron, cobalt, nickel, chromium, tin, Zinc, and the like, may be readily prepared.

While the hydrous metal oxides can be applied to the metal surface in the form of sois, superior results are obtained by precipitating the hydrous metal oxides to produce a fine amorphous hydrous metal Oxide and dispersing the dried precipitate in a non-lubricating Organic liquid. The fine precipitates Can also be dried and used as such in the dried form but normally it is better to employ them in a form wherein they will adhere as a thin film to the metal surface to which they are applied. When they are used in conjunction with a non-lubricating organic liquid the proportions used can be sufiicient to form a homogeneous jelly-like to pasty composition, and in some cases it is desirable to use a sufficient amount of resin or other binder With the fine hydrous metal oxides to form a solid stick which is capable of being applied to the metal surface by rubbing it thereover or by bringing the metal surtace in contact therewith. In general, when finely divided amorphous hydrous metal oxides are incorporated into non-lubricating hydrophilic Or hydrophobic liquids to produce antislip compositions in accordance with the invention, the metal oxides are employed in proportions of about 1 to 50% by weight of the composition, preferably within the range of about to 35% by Weight.

Dried hydrous metal Oxide sols are especially suitable for the purpose of the invention. several diiferent methods may be employed in their preparation. The sol itself is prepared by an ion exchange method as previously described or by some other method such as that described in Patrick, U.S. 2,503,168, and is then dried as such or dried in the presence of an auxiliary substance to facilitate the preparation of a suitable product. For example, 50 ml. of a .5% by weight alumina sol (99.5 water and .5% Al O is evaporated by boiling atfer the addition of 1 to 5 ml. of 0.5 of an aqueous solution of aluminum chloride containing 05% aluminum chloride. The material is subsequently dried. This material is capable of being redispersed in water. In a similar manner other suitable modified alurnina sols are prepared by drying an alumina sol in the presence of a low molecular weight aliphatic alcohol, eg., ethyl alcohol, glycerine, ethylene glycol or diethylene glycol. The dried products are redispersible in water.

Where the hydrous metal Oxide is suspended in a hydrophilic or hydrophobic liquid the invention is not limited to the employment of any particular suspending medium provided it is non-lubricating. The preferred liquids for the purpose of the invention are aliphatic and aromatic TABLE I Viseosity in CDS. at 2G C.

Solvent Name Bronoco Bronoco 75 Bronoco 365 I'Ieptane Mineral Spirits..

Bronoeo Base Oil Brouoco Base Oil Mineral Seal Oil CA) s es e eewwem p OCIDIOOUICIICRCHNOIWOOO Neutral CD Continental Oil Any of the hydrous metal oxides can be dispersed in any of the hydrophobic liquids given in Table I and applied to a metal surface which is brought into contact with another metal surface, preferably after drying the resultant film. While the hydrophobic liquids are preferably hydrocarbons because such substances are more readily available, it will be understood that liquid chlorinated hydro- Carbons such as liquid chlorianted benzenes, ethylene dichloride and other substantiaily inert hydrophobic liquids Can be employed which when mixed with the hydrous metal Oxide produce a jelly-like to pasty composition. In general, non-lubricating Organic hydrophobic liquids having a viscosity at 26 C. between 2.0 and about 8.5 centipoises have given especially advantageous results.

In the drawing a single figure shows a section of a railway rail containing a thin coating of a Composition of the invention applied to the bearing surface thereof.

As shown in the drawing, the rail 1 has a bearing surface Z to which the coating 3 of the antislip composition is applied. The coating of the antislip cornposition may also extend over the surface d which comes into contact with the Wheel flange of the Wheels of locomotives and railway cars.

Evaluation of the Invention 'In order to evaluate the various compositions as agents for improving the Coefiicient of friction between railway car Wheels and rails, the following test apparatus was employed. A siotted wooden holder comprising two strips of wood was mounted on a bench. The holder was so constructed so as to contain a piece of steel four inches long, one inch wide and one-quarter inch thick, herein referred to as a rail. The steel rail Was heat treated and had a tensile Strength of 164200 pounds per square inch, yield point of 159200 pounds per square inch, a percent elongation of 17.3%, and decarburization to a depth of 0.008 inch. An analysis of this steel showed it to contain the following:

Percent by Weight Carbon .3 1 Manganese .58 Phosphorus .0'16 Sulfur .016 Silicon .29 Chromium 1.000 Moiybdenum .222

A Ll-shaped member made of heavy strap steel was formed having two perpendicular pieces attached to the tips of the U. A 94 diameter hole was bored in the Center of the base of the U. A 1 3" diameter steel ball having a Brinell hardness of 500' was welded to a threaded steel rod. The threaded steel rod was placed in the hole formed in the U-shaped steel member and fastened with a nut so that the steel ball was within the Cradle of the U. The perpendicular arms were fitted with small steel boxes Capa-ble of holding lead shot or other weighted material. The U-shaped member was placed on the rail so the steel ball rested on the surface; On either side of the inverted U, wires were attached at a point slightly above the rail surface. Next to the other ends of the wire was a short piece of string attached over a fixed pulley, the top of which was approximately coplanar with the surface of the rail. At the opposite end of the String was a suspended container which Could be filled with weights.

In operation, the boxes were filled with lead shot in an amount which, when included with the weight of the Cradle and fixtures, were calculated to exert a pressure at point of contact on the rail of 73,900 pounds per square inch. The weight of the U-shaped member and ball was 3,065 grams, which, for the purposes of the experiment may be considered as the operative downward pressure. The suspended container was filled gradually with leadshot until the steel ball ;'ust started to slide. This amount of weight is considered as the :force necessary to over- Come the friction existing between the ball and the rail. By using these two factors, the coefl'lcient of friction may readily be evolved from the following simple equation:

Coeflicient of frietion=% where P equals 3,065 grams and F equals the Weight necessary to move the 3,065 grams.

At the start of each series of tests the rail was cleaned with scouring powder, rinsed with distilled water and dried with cellulose tissues. Periodic inspections were made at the Contacting surface and when scratches occurred, the ball and rail were polished with emery paper to renew the surface finish.

To simplify the experimental results the forces necessary to overcome the friction of the steel ball and the rail were recorded in grams. The tests were run by simply smearing a light coating of the composition to be tested over the rail and running the tests while the material was still wet unless otherwise indicated.

In order to establish a 'working basis for comparing the test results obtained, series of tests were run wherein both the rail and the steel hall were perfectly clean. The average of ten such tests showed a force of 1835 grarns necessary to move the ball. When a visible film of journal box oil was applied to the rail the average was 620 grams. All of the tests hereinafter described were conducted with a film of journal box oil at all times initially apparent on the surface of the rail. Hence, readings in excess of 62 indicated compositions of enhanced antislip properties. The following examples illustrate the results obtained and indicate the utility of the invention.

EXAMPLE I A .5% by weight alumna sol was prepared from aluminum chloride in accordance with the process described in Ryznar, U .S. 2438230 This was applied to the oiled rail and when tested wet in two successive tests it gave an average reading of 900 grams. -This reading improved when the Coating of sol was dried.

EXAMPLE II The sol used in Example -I was treated with a caustic alkali to produce a gelatinized precipitate which was applied to the test specimen. When tested wet the average reading was 1287 grams in four passes. An improvement was Obtained in slip resistance when the wet precipitate was dried after being applied to the test specimen.

EXAMPLE III The procedure Was the same as Example I-I except that the concentration of the alumina in the initial sol was 'by weight A1 O The average reading in four passes was 1875 grams when the precipitated alurnina composition was applied to the test specimen and tested wet.

EXAMELE IV A fine alumina having an average particle size of 0.1 to 0.2 micron was dispersed in water to make a 5 sol. Aquantity of sodium hydroxide was added sufiicient to re-precipitate the alumina and the precipitated alumina was applied to the test specimen and dried, Two tests gave an average reading of 1150 grams.

EXMPLE V An alumina sol containing 45% Al O was treated with suflicient sodium hydroxide to produce a uniformgel. The gel was broken up and the moist material applied to the oiled specimen. The average force .in four tests was 116 3 grams. Three more tests were made in which the gel was allowed to dry after being applied but before testing and these tests gave an average reading of 2500 grams.

EXAMPLE VI A fine alurnina (Du Pont) having an average particle size of 0.1 to 0.2 mieron was dispersed in a hydrocarbon solvent (Odorless Base No. 2, Great Lakes Solvents, Inc.) in proportions suflicient to form a jelly containing about 20% by weight of Al O This jelly when applied to the test specimens and tested wet gave an overage reading in three tests of 1950 grams.

In this example, the hydrophobic liquid' employed had a specific gravity at 60 F. of .7936 and an initial boiling point at 387 F., a kauributanol value of 27.0 and an aniline point at 171 F.

EXAMPLE VII A 1.1% ferric Oxide sol was prepared from ferric chloride in accordance with Ryznar, US. 2,438,230, and treated with a small amount of 10% NaOH suficient to produce a gelatinous precipitate. The sol was tested both before and after the treatment with the NaOH. When applied to the test specimen before the treatment with the NaOH it gave a reading of 900 grams when tested wet. The gelatinous precipitate when applied to the oiled specimen gave an average wet reading of 1850 grams in four passes.

EXAMPLE VIII An antislip Composition was prepared by mixing together 27% by weight of precipitated, washed and spray dried catalyst fines and 73% by weight of the hydrophobic solvent described in Example VI. These catalyst fines contained about 13% Al 0 and 87% SiO and had a surface area within the range of 450 to 600 square meters per gram. A good coefficient of friction was developed when this composition was applied to the oiled specimens and tested in the manner previously described.

EXAMPLE IX The catalyst fines described in Example VIII were slurried in Bronoco 365 (an aromatic hydrocarbon sol- Vent) in sufiicient proportions to produce a jelly containing 20% by weight of the catalyst fines. When this composition was applied to the oiled test specimens an average wet reading of 2200 grams was obtained in three successive passes. The test wasrepeated using a new specimen and the average wet reading with this composition was 2167 grams. Another specimen was tested in the usual manner except that the jelly composition was allowed to dry after being applied to the test specimen. The average reading in three successive tests was 2050 grams. The same hydrocarbon liquid without the addition of the catalyst fines when tested according to the general procedure described gave no improvement in the coefficient of friction.

In Examples VIII and IX the catalyst fines can be prepared by several methods. One procedure is to neutraliZe a dilute alkali metal silicate aqueous solution with a dilute aqueous solution of an acid such as sulfuric acid and then impregnate the resultant precipitated silicic acid with aluminum sulfate from which aluminum hyen ar-;re

droxide is precipitated by reaction with aqueous ammonia. The resultant composition is then dried, for example, by spray drying. The fincs are the particles of smaller dimensions which are unsuitable for ordinarily Catalytic use. The proportion of alumina to silica in these compositions will ordinarily vary within the range from 129 to 9:1. Such compositions can also contain other oxides including magnesia, zirconia, titania, thoria, chromium oxides and/ or boron ox'ides.

The invention is not limited to any particular method of applying the tydr'ous metal Oxide to a given metallic surtace forthe purpose of improving the coeficient of friction. Where the hydr'ous metal Oxide is applied to a steel rail, good results can be obtained merely by swabbing the wear baud of the rail with a liquid composition containing the hydrous metal Oxide. The composition containing the hydrous metal Oxide can also be applied directly to the wear band of the Wheel of a locomotive or railway car. lf the hydrous metal Oxide is applied as a wet composition it Can be dried On the surface to which it is applied by means of hot air or in some other suitable manner. However, as shown by the examples, an improvement in the coefficient of friction is obtained even though the composition is still Wet when the Contacting 'metal surfaces are brought together.

In Order to carry out the invention it is only necessary to coat the metal su'riace with a thin film of the antislip composition. Thus, if the antislip composition contains 5 to 50% by weight of a hydrous metal Oxide and it is desired to increase the coefiicient of riction between railway car Wheels and tracks, the arnount of the composition required per mile of two-rail track is within the range of about 1 to 5 gallons applied to the track surface.

While the invention is primarily designed to be applied to the Contacting surfaces of diesel locomotive Wheels and the rails upon which they run, it also has equal effectiveness when it is applied to the Wheels of other railway Car vehicles such as box'car's, passenger ears, and the like. The invention has also equal application in the application to the Contacting surfaces of the Wheels and rails of such forms of transportation as streetcars and cars used in the operation of various types of mines.

The properties or" the hydrous metal oxides have been described herein in terms of surface area and it has been pointed out that the hydrous metal oxides employed for the purpose of the invention have a specific suriace area of at least 25 square meters per grant. Usually the surface area will not exceed 1000 square meters per gram and a preferred range of specific surface area is from about 25 square meters per gram to 600 square meters per gram. The Ultimate particle size or" the hydrous metal oxides will not usually exceed 150 millimicrons in any one dimension. Many of the particles, such as precipitated alumina, are not necessarily spherical but may resemble fibers or tendrils. The reason why these hydrous metal oxides are eflcctive for the purpose of the invention is not clearly understood but is believed to be .attributable in some maner to their fineness, the fact that they are hydrous and capable of absorbing water, and their relatively large surface area.

The term nOn-lubricating as used herein with reference to the liquid employed for dispersing or suspending the hydrous metal Oxide is intended to cover liquids 'which have no substantial lubricating effect. It would be possible, of course, for a liquid to have a slight lubrieating eifect which is more than counteracted by the antirslp properties of the hydrous metal Oxide.

The invention is hereby claimed as follows:

1. The method of improving the frictional contact 'between two metal surfaces capable of motion one with 'respect to the Other which comprises applying to at least one of said surfaces a thin coating of a fine hydrous metal Oxide capable of forming a colloidal sol and having a specific surface area of at least 25 .square meters per gram and bringing surfaees in contact with each other after at least one of them has been so coated.

a 2. The method as Claimed in claim 1 in which the fine hydrous metal Oxide is alumina.

3. The method as claimed in claim l in which the fine hydrous metal Oxide is an Oxide of iron.

4. The method as Claimed in claim l in which the fine hydrous metal Oxide is alumina which is mixed with silica.

5. The method as claimed in claim 1 in which the hydrous metal Oxide is a dried alumina sol.

6. The method as claimed in claim l in which the metal surfaces capable of motion one with respect to the other are respectively a locomotive Wheel and rail.

7. The method of improving the frictional Contact between two metal surfaces capable of motion one with respect to the other which comprises applying to at least One of saic surfaces a thin coating of a suspension of at least 1% 'oy weight of a hydrous metal Oxide capable of forming a colloidal sol and having a specific surface area of at least 25 square meters per gram and an ultirnae particle size not exceeding millimicrons in any dimcnsion in a non-lubricating liquid having a viscosity at 26 C. not greater than 35 centipoises and bringing said surfaces into frictional contact with one another after at least one of them has been so coated.

8. The method as Claimed in claim 7 in which said coating is dried before said surfaces are brought into frictional contact with one another.

9. The method of improving the coeflicient of friction between railway vehicles and tracks which comprises applying to at least one of the Contacting surfaces thereof a thin coating of a suspension of at least l% by weight of a hydrous metal Oxide capable of ferming a colloidal sol and having a specific surface area of at least 25 square meters per gram and an Ultimate particle size in any dimension of not more than 150 millimicrons in a nonlubricating liquid having a viscosity at 26 C. of not greater than 35 centipoises.

10. The method as claimed in claim 9 in which said hydrous metal Oxide is a fine alurnina.

ll. The method as claimed in claim 9 in which said suspension is a mixture of fine alumina and fine Silica and the Weight ratio of alumina to sil-ica is Within the range of1z9 to 921.

12. The method as claimed in claim 9 in which said coating is dried to remove at least a part of said liquid after being applied to at least one of said Contacting surfaces.

13. The method of improving the coeflicient of friction between railway vehicles and tracks which comprises applying to at least one of the Contacting surfaces thereof a thin coating of a suspension of at least l% by weight of a hydrous metal Oxide capable of ferming a colloidal sol and having a specific surface area of at least 25 square meters per gram and an ultirnate particle size in any dimension of not more than 150 millimicrons in a non-lubricating liquid having a viscosity at 26 C. not greater than 8.5 centipoises.

14. A metal structug'le the surface of which is normally brought into frictional contact with another metal surface, at least one of said surfaces being coated with a thin coating of a hydrous metal Oxide capable of forming a colloidal sol and having a specific surtace area of at least 25 square meters per gram.

15 A metal structure the surface of which is normally brought into frictional Contact with another metal surface, at least one of said surfaces being coated with a thin coating of a suspension of at least 1% by weight of a hydrous metal Oxide Capable of forming a colloidal sol and having a specific surface area of at least 25 square meters per gram and an Ultimate particle size in any dimension not exceeding 150 millimicrons in a nonlubricating liquid having a viscosity at 26 C. not greater than 35 centipoises.

16. A railway rail having its Wheel bearing surfaces coated with a thin coating of a hydrous metal Oxide capable of ferming a colloidal sol and having a specific surface area of at least 25 square meters per gram and an Ultimate particle size in any dimension not eXCeeding 150 millimicrons.

17. A railway rail having its Wheel bearing surface coated With a thing coating of a hydrous metal Oxide Capable of ferming a colloidal sol and having a specific surface area of at least 25 square meters per gram and an Ultimate particle size in any dimension not exceeding 150 millimicrons dispersed in a non-lubricating hydrophobic liquid having a viscosity at 26 C. of not greater than 35 Centipoises.

References Cited in the file of this patent UNITED STATES PATENTS 10 Thompson June 3, Fuchs Sept. 29, Stella Oct. 5, Meade Mar. 30, Johannes Nov. 19, Nanfeldt Feb. 11, Baumann et al. Apr. 8, Schaefer May 17, Saul Mar. 28, Stead May 22, Lansing Mar. 19, Luvisi Apr. 9, Lyons Apr. 9, Nohejl Apr. 9, Luvisi Apr. 9, Luvisi Jan. 14, Nohejl Feb. 25, Ryznar Mar. 17, 

13. THE METHOD OF IMPROVING THE COEFFICIENT OF FRICTION BETWEEN RAILWAY VEHICLES AND TRACKS WHICH COMPRISES APPLYING TO AT LEAST ONE OF THE CONTCTING SURFACES THEREOF A THIN COATING OF A SUSPENSION OF AT LEAST 1% BY WEIGHT OF A HYDROUS METAL OXIDE CAPABLE OF FORMING A COLLOIDAL SOL AND HAVING A SPECIFIC SURFACE AREA OF AT LEAST 25 SQUARE METERS PER GRAM AND AN ULTIMATE PARTCLE SIZE IN ANY DIMENSION OF NOT MORE THAN 150 MILLIMICRONS IN 